i think the 700 has a variable level line out instead of a fixed one like the 400. it does have more power. i don't think you can make use of the 50W horn amplifier unless you get a GK rbh (i think-the ones with the horns) series cab. other than that, the tone is the same. more headroom is good though. i dunno what your money situation is, but i would probably get the 400 and then put the extra $ into a cab...

The 700RB is pretty different from the 400RB. They are NOT the same amp with different amounts power. The circuitry in the 700RB is different from that in the 400RB.
On the other hand, the 1001RB is the same amp as the 700RB, except more powerful.
I would spring for the 700RB, if you can afford it. The tone is a little more refined, and you have more patching and voicing options. As you start playing with louder guitar players and drummers you will appreciate the extra power, too.